The present invention relates to calibration phantoms for x-ray machines and the like and, in particular, to a phantom providing calibration of dual energy x-ray equipment for percent body fat measurements.
The absorption of x-rays by material is dependent on two independent absorption mechanisms: Compton scattering and the photoelectric absorption. Each of these absorption mechanisms differs in relative contribution for different materials. Further, the amount of absorption by each of these mechanisms is dependent on the energy of the x-rays. Measurements of the attenuation of a body at two x-ray energies (generally corresponding to different x-ray frequencies) can therefore reveal information about the relative contribution of Compton scattering and the photoelectric absorption by materials of the body and thus can reveal information about the constituent materials of the body.
One important use of dual energy measurement is in determining relative proportions of bone and soft tissue in a patient providing information about the patients bone mass. Lunar Corporation, the assignee of the present application, manufactures a number of dual energy x-ray machines suitable for this purpose as described in the following U.S. patents hereby incorporated by reference: U.S. Pat. Nos. 5,253,282; 5,228,068; 5,287,546; 5,291,537; 5,305,368; 5,306,306; 5,408,439; 5,485,492; 5,509,042; 5,533,080; 5,533,084; 5,577,089 and 5,673,298.
If the basis materials are chosen to be tissue and fat instead of tissue and bone, the same techniques may be used to provide a measurement of percentage body fat as may be useful in the investigation of drugs and in the study of wasting diseases.
In either of these applications, it is important that repeatable quantitative results be produced. For this reason, the entire patient may be scanned so as to eliminate variation caused by change in measurement regions for different patients or the location of the measurement region between measurements of a single patient. Further, the machine must be calibrated both to prevent drifting of the measurements of a single machine over time and, in the case where multiple machines are used in a study, to ensure consistency among results. Critical to this calibration is that the machines produce identical readings for the same patient at a given time regardless of the patient's body fat. Given that a total body scan will typically be used, it is preferable that the calibration process reflect measurements over a substantial area on the machine.
What is needed is a simple and reliable phantom that allows accurate calibration of dual energy equipment for body fat measurement under these constraints.